1. Field
This invention pertains to compound archery bows, and is more particularly directed to the eccentric members associated with the flexible limbs of such bows.
2. State of the Art
Archery bows of the type commonly known as "compound bows" are generally characterized by a pair of flexible limbs extending from opposite ends of a handle. The tips of the limbs are thus spaced apart in relationship to each other in a fashion similar to the limb tips of a traditional stick bow. The limbs are deflected by the operation of a bowstring in the same fashion as a traditional bow, but the bowstring is interconnected to the limbs through a rigging system including mechanical advantage-varying structures (usually sheaves, such as those commonly referred to as "eccentrics") and tension runs which transfer a multiple of the bowstring tension to the respective limbs. Tension runs are interchangeably and loosely referred to by those skilled in the art as "cables," "cable stretches," "bowstring end stretches" and "end stretches." In any event, the rigging system may be regarded as a block and tackle arrangement whereby pulling force applied to the bowstring (at its nocking point) is transferred to the limb tips to flex the limbs. The bowstring and tension runs may comprise a single continuous loop, but more typically, the bowstring is constructed of special bowstring material, while the tension runs are of more rugged construction, e.g. as from aircraft cable. The bowstring and tension runs together are referred to interchangeably as the "cable system," "cable loop" or "rigging loop."
The rigging of a compound bow functions as a block and tackle to provide a mechanical advantage between the force applied to the bowstring by an archer and the force applied to the bow limbs. In other words, in operation, the nocking point of the bowstring is moved a longer distance than the total distance that the two limb tips move from their braced position. Although other configurations are possible, an eccentric sheave is usually pivotally mounted at each limb tip. If the eccentrics are mounted elsewhere, the rigging usually includes a concentric pulley at each limb tip.
The term "sheave" is intended in this disclosure to include any structure mountable to rotate about an axis to present a winding surface analogous to a pulley, wheel or drum. The term thus includes elements common to the structures known within the archery art as compound bow "eccentrics," "cams," "wheels" and the like. As so defined, an eccentric may include one or more sheaves, each of which includes one or more "grooves." Depending upon their individual functions, these grooves are called either "string grooves" or "cable grooves." The grooves are regarded as being provided in the surface of the sheave, although it sometimes occurs that the depth of a groove may increase as it progresses about the axis of the sheave so that the bottom of the groove approaches the axis more rapidly than does the outer surface of the sheave. The string grooves of an eccentric comprise a "string track," and the cable grooves comprise a "cable track."
Each sheave of an eccentric thus has grooves or tracks analogous to the pulley grooves in a block of a traditional block and tackle. A string track is arranged alternately to pay out or take up string (or the portion of the cable directly linked to the string) as the limbs are alternately flexed to drawn or relaxed to braced condition. A cable track is arranged alternately to take up portions of the tension run (as string is paid out while the eccentric pivots to drawn condition) and to pay out portions of the tension run (as string is wound onto the string track while the eccentric pivots to braced condition). In some instances, either the cable sheave or the string sheave may actually be concentric with respect to the axle of the eccentric.
For purposes of this disclosure, it is recognized that in the operation of a compound bow, the portion of the rigging called the bowstring actually lengthens as the string is pulled back because as the eccentrics pivot from their braced condition, portions of the bowstring stored in the string tracks unwind and are paid out. Concurrently, portions of the tension run are wound onto the cable tracks of the eccentrics so that the tension runs descrease in length. The opposite phenomenon occurs as the string is released, permitting the eccentrics to pivot back to their braced condition. Assuming that the eccentrics are carried by the respective limb tips, the portion of the rigging loop extending between points of tangency of the bowstring with the string track of the eccentrics will be referred to herein as the "central stretch" of the bowstring. The bowstring shall be considered to include, in addition to the central stretch (sometimes called the "working stretch"), portions of the rigging loop stored at any time in association with the string tracks of the eccentrics. The portions of the rigging loop extending from the points of tangency of the tension stretches with the cable tracks of the eccentrics to remove points of attachment to the bow shall be called the "end stretches." Each tension run is considered to include, in addition to an end stretch, the portion of the rigging loop extending from the end stretch and wrapped within or otherwise stored in association with the cable track of the associated eccentric.
In an archery bow of the type commonly referred to as a "compound bow," the shape of the force-draw curve is determined in substantial part by the "cam ratio" of the rigging as a function of draw (movement of the nocking point from its at rest position). The "cam ratio" is defined as the ratio of the distance measured from the axis of an eccentric of the rigging to the tangent point of contact of the bowstring with the string groove to the corresponding distance between the axis and the tangent point of contact of the cable with the cable groove of that eccentric. Various combinations and configurations of string grooves and cable grooves are disclosed, for example, by U.S. Pat. Nos. 3,486,495; 3,958,551; 4,060,066; 4,337,749; and 4,338,910. These configurations influence the course of the "cam ratio" of the rigging of a bow as the nocking point is pulled from braced condition to various drawn positions.
Each bow may be described by reference to a specified "draw length," at or near the "valley" of its force-draw curve. Heretofore, it has not been conveniently available to adjust the draw length of a compound bow without indirectly affecting the location (draw position) and/or magnitude of the peak draw force of the bow. Adjustments to draw length have conventionally involved adjusting the rotational positioning of the eccentrics on their axes. The course of the "cam ratio" of the rigging as a function of draw has inherently been simultaneously affected. Adjusting the draw length of a bow has also necessitated loosening the rigging and restringing the bow.
Eccentrics have recently become available which include interchangeable segments. These segments may be used to restructure the eccentric to reprogram the course of the cable track, thereby adjusting the draw length of a bow outfitted with such eccentrics. The segments may be changed with the bow in strung condition. It would be an important advancement in the art to provide a rigging for a compound bow permitting independent adjustment of draw length and/or "let-off" and peak draw force. As a corollary, it would be a significant benefit to be able to adjust the draw length of a compound bow (providing for approximately minimum post peak draw force at an individual archer's "full draw" length) while the bow is in its strung condition.